This invention is directed to a step motor drive control system and, in particular, to a closed loop driving arrangement for driving a step motor in incremental steps.
In general, closed loop control of step motors is a preferred method for single-step driving of step motors. This is due to the fact that open loop control of step motors cannot detect the angular position of the rotor of the step motor or the speed and acceleration of the step motor. Two basic types of systems for driving step motors are available. In the first type of driving system, the driving system first actuates certain phases (as used herein, the term "phase" refers to the manner in which the coil or coils of the step motor are actuated to effect driving) to drive the step motor, deactuates the phases upon elapse of a predetermined interval of time, and then reactuates the phases which were first actuated upon elapse of another predetermined interval of time. In the second type of driving system, the driving system drives the step motor in micro-steps by changing the ratio of a current in each phase in order to subdivide each step of the motor into micro-steps for controlling the motor with a resolving power of such micro-steps. The second type of driving system is utilized where coils of two or more phases are actuated as with two-two phase excitation of a step motor.
According to the first drive system described above, the time intervals for actuation and deactuation are predetermined by way of experimentation to thereby determine a sequence of actuation times. When the step motor is subjected to load variations and varying power supply voltages, the motor tends to vibrate greatly and undergo step-out thereby failing to achieve good step response characteristics. Such a system is also disadvantageous in that the force with which the motor can be braked for deceleration is mostly a frictional force load imposed on the motor and, hence, is small, resulting in an increased period of time required to decelerate the motor. This is a closed loop system. A closed loop speed control system for step motors used to drive print heads of printing devices is disclosed in U.S. patent application Ser. No. 06/244,936, filed Mar. 18, 1981 and assigned to the same assignees as the present application.
According to the second driving system referred to above, open loop drive control of the step motor is achieved. However, with one step of a step motor being divided into smaller substeps for controlling the step motor with a resolving power of such substeps, the vibrations caused due to motor load variations and voltage differences are more reduced the greater the resolving power of the steps become. Such a system, although having good step response characteristics, is nevertheless disadvantageous in that there are required as many circuits for changing the current flowing through the phase coils, respectively, of the step motor by the steps of the resolving power for the motor as there are phases, and hence the overall circuit is complex and quite costly. The present invention has been designed in an effort to eliminate the difficulties described above. Accordingly, a step motor drive control which accurately drives a step motor with excellent single-step response is desired.